E.C. Niculescu

Donor impurity in a finite parabolic quantum well

Abstract A variational calculation within the effective mass approximation for the states of single and double donors in a finite parabolic quantum well is presented. The results show that the effect of the finite well is to lower the binding energies, especially for the ground state of double donor centers. The validity of the infinite-parabolic-well approximation is also discussed.

Binding energy of a double donor in a parabolic quantum well

Abstract We have calculated the binding energies of the ground state of a double donor associated with the first subband in a parabolic quantum well. Taking into account the nonparabolicity of the conduction band, we have considered the two cases of positive donor center and neutral donor center. The results have been obtained as a function of the parabolicity parameter by using a variational approach. It is found that the nonparabolicity causes an increase in the binding energy; also, this energy increases with the increasing of the parabolicity parameter α.

Energy spectra of donors in spherical quantum dots with parabolic confinement

The binding energy of a hydrogenic donor in a spherical GaAs-Ga1−xAlxAs quantum dot with parabolic confinement is calculated by taking into account the finite value of the barrier potential. Using the effective-mass approximation, we obtained the energy of the ground state as a function of the dot size for different impurity positions. The results show that the effect of the finite value of the barrier potential is to lower the energies of the states. The differences between the proper solutions and the infinite barrier potential approximation increase noticeably, especially for on-center impurity and for small dot radii. We have also shown that the impurity binding energy is much dependent upon the impurity position in the system. This fact is important for a correct description of impurity-related absorption and photoluminescence experiments.

Effect of electric field and central-cell corrections on the spectrum of shallow donors in parabolic quantum wells

The effects of the electric field and of the central-corrections on the binding energies of shallow donors in a Ga As/Ga1−xAlxAs parabolic quantum well are studied. The effectivemass approximation within a variational scheme is adopted, and central-cell corrections are calculated by using a model potential with an adjustable parameter. For great values of the parabolic parameter, relatively large corrections are obtained for the shallow donors studied.

Electric field effects on the magnetoexcitons in quantum wells

In the framework of the effective-mass envelope-function theory, the electronic properties of a rectangular quantum well (QW) in the presence of a strong magnetic field in the QW plane and a crossed electric field parallel to the growth direction are studied. The single particle states for electrons and holes and the excitonic binding energy are obtained for several magnetic-field values, as functions of the electric field strength. The electric field breaks down the degeneracy of the states symmetrically positioned in p space, leading to a nonparabolic subband structure. We found that for the exciton moving in the QW plane, for some applied field strengths, (i) the binding energy exhibits a maximum and (ii) a transition of the ground state from a zero in-plane centre of mass momentum to a finite momentum occurs.

Effect of electric field on the energy levels of a double donor impurity in a square quantum well

Abstract The binding energy of a double donor at the centre of a GaAs/Al x Ga 1− x As square quantum well subject to a perpendicular electric field is calculated by a variational method. The results show that the electric field significantly modifies the deep levels of the double donor for well widths from 50 to 200 A.

EXCITONS IN A NARROW PARABOLIC QUANTUM WELL UNDER EXTERNAL FIELDS

The effects of electric and magnetic fields on the ground (1S-like) and excited (2S-like) states of an exciton in a narrow GaAs/AlxGa1-xAs parabolic quantum well are studied. The effective-mass approximation within a perturbation-variational scheme is adopted. We find that the hole-mass anisotropy and nonparabolicity of the conduction band significantly modify the electron properties in such structures in which the quantum confinement plays a fundamental role. The effect of the electric field on the spatial distribution of the electron and hole is also investigated. In the low field regime, the diamagnetic shift of the exciton energies is calculated.

Double donor in a spherical quantum dot

We present a variational calculation for the ground state of the double donor in a spherical GaAs–Ga1−xAlxAs quantum dot. The binding energies for the ionized and neutral centres are calculated for several barrier height values as a function of the radius of the dot. Compared with a square well structure, there is a stronger confinement and a larger binding energy for the double donors in a spherical quantum dot.

Optical non-linearities associated to hydrogenic impurities in InAs/GaAs self-assembled quantum dots under applied electric fields

Abstract The effects of the hydrogenic impurity on the electron-related non-linear optical processes in a InAs/GaAs dome-shaped quantum dot with a wetting layer under applied electric fields are studied within the density-matrix formalism. The one-electron energy levels and wave functions are calculated using the effective mass approximation and the finite element method. The non-linear optical absorption, relative refractive index change and non-linear optical rectification associated with interlevel transitions are calculated under a strong probe field excitation for both in-plane and z-polarisation of the incident light. According to our results as the electric field increases the absorption and dispersion peaks decrease and exhibit red shift. Hydrogenic impurity located at the origin induces a blue shift in the optical responses. For the optical absorption coefficient the peaks magnitude is enhanced by the impurity presence independent of the electric field strengths, whereas the non-linear optical rectification is larger in the case with impurity only for zero applied electric field.

Shallow acceptors in a quantum well under intense laser fields

In this paper we have studied comparatively the effects of the high-frequency laser field and impurity position on the transition energy associated with a simple neutral acceptor and with a singly ionized, double acceptor in GaAs/AlGaAs quantum wells. A blue shift of the transition energies when the laser field intensity increases is predicted.